Let antigen-selfupdate works when in submodule

When antigen is in submodule (ex: antigen is a submodule of dotfiles repo), the `.git` will be a regular file instead of directory. In this case, `antigen-selfupdate` refuses to update itself because there is no `.git` directory. This patch changes the condition to `-d .git || -f .git` so that `antigen-selfupdate` can work when antigen is a submodule. Also, `git checkout master` is added because submodule will detach from master branch.